There is a need in many industries for the measurement of thread characteristics on screws, bolts and other fasteners and components. Apparatus for this purpose may be broadly categorized into contact and non-contact approaches. Contact-type gages typically employ feelers which contact the threaded surface to be measured at set points and mechanically measure the tolerances. One disadvantage of such devices is that the contact feelers can become worn with usage or become out of adjustment. Also, because the gage only takes a reading when the feeler is contacting the component, 100% coverage of the threaded surface during the inspection is not possible.
Numerous non-contact thread measurement techniques have been developed, including systems that employ lasers and video cameras. U.S. Pat. No. 4,315,688 shows an apparatus for inspecting threaded objects, such as bolts, moving on a conveyor line past the inspection station. A light-sensitive detector picks up reflected light and produces an analog output which is used to determine the quality of the threads. The light-sensitive area of the detector is sufficiently small to resolve the individual threads of the threaded object, to determine whether the correct number of threads are present.
U.S. Pat. No. 4,598,998 discloses a screw surface flaw inspection method and an apparatus therefore. The system projects light onto the surface of a screw, the light being scanned axially of the screw. A detector picks up the reflected light from the projected surface and sends an output signal to a signal processing circuit to thereby detect the flaw on a basis of time base variation of the intensity of reflected light. The method and apparatus enables the inspection of minute flaws rapidly and exactly.
The apparatus of U.S. Pat. No. 4,644,394 has a light source for illuminating the threaded surface to be inspected, a mirror system for directing the light from the light source, and a video camera for receiving the directed light, for forming an optical image of the illuminated threaded surface, and for converting the optical image into electrical video signals. An encoding means converts the video signals from the camera into digital information representative of special information in the optical images viewed by the video camera. Processing means receive and interpret the digitized information provided by the encoding means for analyzing the thread characteristics and for detecting defects in the threaded surface being inspected.
An apparatus for measuring the profile of portions of an article located within a predefined plane is disclosed in U.S. Pat. No. 4,906,098. Each portion is scanned, such as by an optical micrometer providing a beam of radiant energy, to determine its dimension. The distance between each portion and a vertical reference is also scanned to determine its dimension. The article is rotated about an axis intersecting the predefined plane within the scan of the beam and is axially moved along an axis parallel to the intersecting axis within the scan of the beam so that the dimension of each portion and its distance from the vertical reference can be determined. The apparatus may be used in combination with a cavity identification system to control manufacturing employing multiple molds.
U.S. Pat. No. 5,521,707 uses laser triangulation to quickly build a precise profile of a thread form. The sensor is mounted on a precision mechanical system that moves the sensor to scan the thread form, producing a set of digitized images of a thread form that are digitally stored. The digitized images are analyzed to derive quantitative information about thread characteristics such as pitch, lead, root radius, flank angle, surface roughness, helix variation, and pitch diameter. Thread characteristics may be stored and later retrieved in order to provide traceability and verification of the part.
U.S. Pat. No. 5,608,530 utilizes a laser for producing a beam of radiation which is then refined in cross section through use of plano-cylindrical lenses. The refined beam of radiation falls incident on a part to be measured. The unobstructed portions of the beam are then redirected by a pair of reflective surfaces producing non-parallel radiating beams; each beam comprises of the unobstructed portion of radiation which has passed radially opposed halves of the part. The magnitude of radiation present in each non-parallel radiating beam is then measured. The magnitude of radiation measured is proportional to a dimensional measurement. However, the assumption must be made that the part is placed perfectly in the center relative to lens; if not, diameter measurement may be problematic. In addition, since the resolution limit is a strong function of laser wavelength, performance may suffer if component layout is not properly aligned. For example, if the light from the laser varies, this will result in a change at the detectors which, in turn, could be misinterpreted as a diameter irregularity.
Thus, despite the advances of these and other approaches, the need remains for simple yet effective inspection apparatus and method.